In the art concerning animal feed it is a well known fact that pelleting of feed is a desideratum, as pelleting of feed increases the digestibility of especially the starch fraction of the feed. Furthermore, it is known that pelleting of animal feed reduces dust problems.
In the process of producing feed pellets it is considered necessary to steam treat mash feed in order to kill pathogenic microorganisms if present and to partly gelatinize starch in order to improve the physical properties of the pellets, whereby a steam treatment of around 70-120° C. is appropriate. Active compounds present in the feed pellets such as enzymes are not very stable at high temperature or humidity, and thus, a large surplus of enzymes has to be used, or enzyme free feed components are pelletized and steam treated, where after an enzyme containing slurry or solution is coated onto the steam treated pellets. However, this coating process is cumbersome and is often not compatible with feed mill equipment. An attempt to obtain improved enzyme granules for feed is found in WO 92/12645. WO 92/12645 describes T-granules, which are coated with a fat or a wax. Said T-granules are mixed with feed components steam treated and subsequently pelletized. By this invention it was possible to heat treat the granules comprising enzymes and avoid the cumbersome coating with enzymes after the heat treatment. The use of wax coated T-granules was a significant improvement in this field as it was possible to maintain an acceptable enzyme activity during steam pelleting. Another attempt to improve the pelleting stability of enzyme granules is described in WO 2006034710 where it has been found that coating enzyme granules with a salt coating improves the pelleting stability. However some feed mills are run under very aggressive conditions which are still very harsh for the improved enzyme granules, thus there is still a demand for improved pelleting stability. Furthermore the drawback with coating of the enzyme granules is an additional process step.
The present invention provides very good pelleting stability and makes the step of coating optional.
It is described in WO 99/32595 to use copper sulphate as barrier coating in enzyme granules to stabilize enzyme granules for detergents. It is also known in the art to use inorganic salts of zinc and magnesium to stabilize enzyme granules, see WO 97/05245.